> By the way, since you need to apply thrust from any direction to maneuver flexibly in 3D space (without wind or gravity), wouldn't spherical/disc(saucer) shaped spacecraft be the most efficient?
Definitely not. There's very little need to 'maneuver flexibly in 3D space'. You are mostly interested in rotation. A slight amount of translation if you are trying to dock with something else.
You will likely still prefer to thrust mostly in a 'forward' (wherever forward is) direction. If, for some weird reason, you wanted to have thrusters equally powerful in all directions, just imagine the amount of weight (and plumbing) this would require.
Definitely not. There's very little need to 'maneuver flexibly in 3D space'. You are mostly interested in rotation. A slight amount of translation if you are trying to dock with something else.
You will likely still prefer to thrust mostly in a 'forward' (wherever forward is) direction. If, for some weird reason, you wanted to have thrusters equally powerful in all directions, just imagine the amount of weight (and plumbing) this would require.